The present invention relates generally to testing of hydraulic components and, in particular, to air testing vehicle hydraulic brake components such as those found in vehicle anti-lock brake and/or traction control systems.
Anti-lock brake and traction control systems are becoming increasingly popular with purchasers of vehicles. Generally, anti-lock brake systems include sensors for monitoring the speed of the controlled wheels and a Central Control Unit (CCU) to determine when an impending wheel lock-up condition is present during vehicle braking. When a predetermined slippage has been detected at the controlled wheels, the CCU functions to control the application of hydraulic pressure to the associated brakes to prevent lock-up of the controlled wheels.
Typically, the Anti-lock Brake System (ABS) includes solenoid actuated valve means for cyclically relieving and reapplying hydraulic pressure to the associated brakes. The valve means operate to limit wheel slippage to a safe level, while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver. When reapplying pressure, the valve means are supplied pressurized hydraulic fluid from a positive displacement hydraulic pump or the brake system master cylinder. The ABS can also include hydraulic fluid storage reservoirs, or accumulators. Check valves can also be included to control the direction of flow of hydraulic fluid within the system. Traction Control Systems (TCS) typically include additional valves for applying braking force to reduce wheel spin upon a slippery surface and thereby redistribute the power being delivered between the driven wheels.
Typically, the solenoid valves, pump, accumulators and other components, or subassemblies, are assembled in an ABS and/or TCS Hydraulic Control Unit (HCU). The HCU includes internal passageways which provide communication between the different system components contained therein. External ports formed in the HCU allow connection of hydraulic lines for communication with a conventional brake master cylinder and the wheel brakes.
It is known in the art to functionally test assembled HCUs to confirm correct assembly thereof before shipping to vehicle manufacturers. The functional tests also are designed to assure that the HCUs conform to operational specifications. Such prior art tests typically consist of mounting an assembled HCU upon a test stand, filling the HCU with hydraulic fluid, connecting the HCU external ports to test equipment and electrically connecting the HCU to a CCU. The CCU exercises the HCU components to simulate anti-lock braking or traction control cycles while various hydraulic pressures are measured by the test equipment and compared to specified limits.
Functional tests utilizing hydraulic fluid as a test medium tend to be time consuming, requiring evacuation of all air from the HCU to assure accurate test results. It is necessary to control the hydraulic fluid temperature, avoid aeration of the hydraulic fluid and prevent fluid contamination with small particles of dirt or metal.
Functional testing using hydraulic fluid involves a closed test loop during which the fluid is recirculated within the HCU. Under these conditions, it has been found that the CCU can modify the performance of one component in the HCU to compensate for another slightly defective component. This can effectively mask a defect which potentially can develop into a more serious problem during operation of the HCU on a vehicle. In a similar manner, the CCU also has, under test conditions, compensated for incorrectly sized components that were mistakenly installed in the HCU. Furthermore, it has been found difficult to remove all hydraulic fluid from the HCU after testing for shipping the unit to the vehicle manufacturer. Residual fluid in the HCU will tend to cause evacuation and fluid fill problems at the customer's assembly plant.